Process for making nitrocellulose pro-



United States Patent Ofifice PROCESS FOR MAKING NITROCELLULOSE PRO-PELLANT FORMULA'HQNS HAVTNG lMLPRQVED TENSELE STRENGTH AND BALLKSTHCPRO?- ERTIES Eli D. Bessel, China Lalre, Robert T. Mather, Ridgecrest,and William G. Finnegan, China Lake, 'Lalif assignors to the UnitedStates of America as represented by the Secretary of the Navy N Drawing.Filed .ltrly 25, 1957, Ser. N 674,261

4 Claims. (Cl. 149-98) (Granted under Title 35, U.S. Code (i952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to a process for making nitrocellulose-basepropellent formulations having improved ballistic properties andincreased strength.

It is now understood in the art of propellants that the term double basepropellant means a nitrocellulose-nitroglycerin propellant. A mesadouble base propellant is one which produces mesa characteristics uponburning; the mesa effect as well as mesa type double base propellantsbeing fully disclosed in application Serial No. 352,312, Gas ProducingCharge, by Albert T. Camp, filedin the U.S. Patent Oliice on April 30,195 3, and given a notice of allowability on February 17, 1955. Asexplained therein, an indicative factor of the effective operation of adouble base rocket propellant is the slope n, or pressure exponent, ofthe curve produced by a logarithmic graph of the burning rate of thepropellant plotted against pressure produced during burning. This factoris particularly important in the zone of useful rocket pressures,roughly, 700 to 3000 psi. The relationship between the pressure at whicha propellant burns and its burning rate is mathematically expressed asr=cp or as log r=n log px-log c, where r is the burning rate, p is thepressure at which the burning rate is measured, and c and n areconstants characteristic of a given propellant. A plot of log r againstlog p for conventional propellants produces a straight line of slope n,that is, there is a progressive increase in burning rate for eachincrease in pressure. This characteristic is disadvantageous inpropellants for jet actuated devices wherein it is highly desirable tohave the value of n as low as possible and certainly below .7 in thezone of useful rocket pressures. For example, variations of performancedue to changes in temperature, burning rates and operating pressures ofprior propellants in the zone of useful rocket pressures necessitatedexpensive fire control apparatus designed to correct for theseundesirable results. Further, heavy walled rocket motors were requiredto withstand the very high pressures developed at high temperatures offiring when prior art propellants were used, and the propellants werehighly inefficient because of the low pressure and thrust developed withthem at low temperatures of firing. Research developed that the additionof small amounts of additives such as certain lead compounds referred toas ballistic modifiers produced a plateau type propellant for which thevalue of the pressure exponent n is zero. Further research resulted inthe mesa type propellant which is superior in performance in rocketmotors to the plateau type propellant. Mesa type propellants arecharacterized by the fact that the slope in of the curve representingtheir pressure-burning rate relationship becomes zero at some point andthen reaches a negative value thereafter in the region of useful rocketpressures, that is, the burning rate in this negative slope regionactually decreases with increase in pressure.

A rocket propellant exhibiting mesa characteristics provides a number ofhighly desirable effects. For example, the negative pressure exponentserves as a safety valve in 3,103,458 Patented Sept. 10, 1963 case ofsudden large changes in burning surface during the operation of arocket, such as that caused by cracking of the grain. With prior artpropellants such a failure of the grain would ordinarily result indestruction of the rocket motor, but in the mesa type propellants only asmall pressure increase results. Further, there is an inherent tendencyfor overlapping of ratepressure relationships at various temperatures asillustrated by logarithmic graphs of the relationships, that is, incertain regions of pressure the burning rate of propellant for firingsat low temperature may actually be higher than the burning rate forfirings at high temperature. Also, the variation in performance withchange in temperature for mesa type propellants is negligible and insome cases there is none at all. This advantage is particularly usefulas respects fire control considerations in the design of aircraftrockets to be fired at moving targets. Further, the burning rate andenergy content of the mesa propellants are higher than those of priorart propellants and can be controlled at will over rather wide limits.

A great deal of research is being directed to the development ofimproved stabilizers, plasticizers, ballistic modifiers and othercomponents of mesa type propellants. A prime consideration in thedevelopment of such components is the fact that their addition to thefinal propellent composition must not vitiate the highly sensitive mesaeffect of the propellant and, preferably, the presence of the additivesshould enhance the mesa effect.

A major difficulty in the development of nitrocellulosebase propellantsis the production of a propellant grain possessing sufficient strengthto withstand handling and long storage periods without cracking, andwhich will not crack or fissure during burning with resultant erraticrocket performance. It has long been recognized that the process bywhich the propellant is compounded can :be a controlling factor inobtaining high-strength grains.

It is therefore an object of this invention to provide a process for thecompounding of nitrocellulose-base propellent formulations of increasedstrength and enhanced ballistic properties.

The invention comprises homogeneously mixing the nitrocellulose-basepropellant ingredients with an amount (about of the nitrocelluloseweight) of a mixture of acetone/ethyl alcohol in about 65/ 35 weightratios, respectively, and adding diethyl ether in an amount equal to 25to of the Weight of the nitrocellulose. The formulation is then extrudedat ambient temperature and dried. A modification of the process by whichthe necessity for the addition of diethyl other is eliminated consistsof the extrusion of the propellant formulation at a temperature betweenand F.

The following examples of propellant formulations were processed by awell established slurry process of the prior art and by the process ofthe invention. The slurry process is the one used for making N-Spropellant and is as follows: The nitrocellulose is mixed to a thinslurry in about 10 times its weight of warm Water and finely groundZ-nitrodiphenylamine added slowly. A solution of lead 2-ethylhexoate in2-3 times its weight of warm diethyl phthalate is admixed with theslurry. A solution of the nitroglycerin in the remaining diethylphthalate to be used is slowly added to the slurry. Thereafter, theslurry is filtered or centrifuged to remove most of the water and theresulting paste is aged for a period of 1 to 5 days or more at atemperature of about 130 F. It is then dried to a moisture level of from8 to 15 percent, the lead salicylate added to the partially dried pasteand the mixture milled to a homogeneous colloid on a heated differentialrolling mill. The method of mixing is not critical, provideddistribution of all ingredients is uniform and no losses of ingredientsoccur o which are not otherwise accounted for. The sheet propellantformed may be extruded into strands or other forms in a warm press. Themixing operation may also be done by a damp process in conventionalSchroeder bowl or Sigma blade, or Talley mixers. In this case the leadsalicylate is preferably added to the damp nitrocellulose before theother ingredients. The object is to achieve optimum homogeneity;Slightly modified procedures are equivalent provided they do not causemaldistribution of any ingredients. Comparative strength tests made onstrands of propellant processed by the two methods are given. In

'rnaking the formulations by the process of the invention,

the propellant ingredients were thoroughly mixed with an amount (about80% of the nitrocellulose weight) of a mixture of acetone-ethyl alcoholin about 65/ 35 weight ratios, respectively. To the formed mixture wasadded diethyl ether in an amount equal to 25 to 100' percent of theweight of the nitrocellulose. The formulation was extruded at ambienttemperature into strands A -inch in diameter and the strands allowed todry. The examples are intended to be illustrative of the invention butnot limiting thereof. 40 and 200 gram batches were made up having thefollowing percentage compositions:

The strands were tested with a standard tensile strength testingmachine.

Typical strength values for the above compositions are as follows:

STRANDS MADE BY THE SLURRY PROCESS FOR MAKING N- PROPELLANT I II III IVTensile Strength (p.s.1.) 600 600 800 800 STRANDS MADE BY THE PROCESS OFTHE INVENTION Tensile Strength (p.s.i.) 1,000 2,500 1,500 4,000

dinitriles which were tested by the process of the invention and themodification thereof. Dinitriles, as a class,

display a high solvating action on nitrocellulose. When formulationscontaining dinitriles are processed by the present process thepropellant matrix is translucent rather than opaque. There is someevidence that the dinitrile plasticizer complexes with thenitrocellulose and nitroglycerin. The results also show that the effectis more pronounced when higher percentages of nitrocellulose are used inthe formulations.

Equally striking comparative results were obtained when the abovereferred-to modification of the process was used; that is, when the stepof adding diethyl ether was omitted and the propellant formulation wasextruded at a temperature between 130 and 150 F. In compoundingformulations by the modification of the process the compositions ofExamples 1 through 4, inclusive, were used.

In addition to the production of higher strength formulations it wasfound that a double-base propellant is produced by the new processhaving sharper and more pronounced mesa burning characteristics.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that Within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A process for making double-base propellant grains which contain fromabout 40% to about nitrocellulose and from about 3045% nitroglycerin andat least 5% of a dinitrile containing up to ten carbon atoms whichconsists essentially of thoroughly mixing the propellant ingredientswith an amount equal to about of the weight of the nitrocellulose, of amixture of about 65 parts of acetone and about 35 parts of ethylalcohol, by weight, adding diethyl ether to the formed mixture in anamount equal to about 25 to about of the weight of the nitrocellulose,extruding the resulting formulation into the desired shape, and dryingthe extruded product.

2. The process of claim 1 in which the step of adding diethyl ether isomitted and the mixture extruded at a temperature from about to about F.

3. The process of claim 1 in which the propellant to be extrudedcontains, in addition to the ingredients enumerated, about 2%Z-nitrodiphenylamine, about 1.2% lead salicylate, and about 1.2% leadZ-ethylhexoate.

4. The process of claim 2 in which the propellant to be extrudedcontains, in addition to the ingredients enumerated, about 2%2-nitrodiphenylamine, about 1.2% lead salicylate, and about 1.2% lead2-ethylhexoate.

References Cited in the file of this patent UNITED STATES PATENTS1,978,070 York Oct. 23, 1934 2,153,331 Kunz Apr. 4, 1939 2,159,208Goodyear May 23, 1939 2,401,236 Fielitz May 28, 1946

1. A PROCESS FOR MAKING DOUBLE-BASE PROPELLANT GRAINS WHICH CONTAIN FROMABOUT 40% TO ABOUT 65% NITROCELLULOSE AND FROM ABOUT 30-45%NITROGLYCERIN AND AT LEAST 5% OF A DINTRILE CONTAINING UP TO TEN CARBONATOMS WHICH CONSISTS ESSENTIALLY OF THOROUGHLY MIXING THE PROPELLANTINGREDIENTS WITH AN AMOUNT EQUAL TO ABOUT 80% OF THE WEIGHT OF THENITROCELLULOSE, OF A MIXTURE OF ABOUT 65 PARTS OF ACETONE AND ABOUT 35PARTS OF ETHYL ALCOHOL, BY WEIGHT, ADDING DIETHYL ETHER TO THE FORMEDMIXTURE IN AN AMOUNT EQUAL TO ABOUT 25 TO ABOUTT 100% OF THE WEIGHT OFTHE NITROCELLULOSE, EXTRUDING THE RESULTING FORMULATION INTO THE DESIREDSHAPE, AND DRYING THE EXTRUDED PRODUCT.